An Algorithm for Dissemination and Retrieval of Information in Wireless Ad Hoc Networks

Transcription

1 An Algorithm for Dissemination and Retrieval of Information in Wireless Ad Hoc Networks Euro-par 27 Hugo Miranda Simone Leggio 2 Luís Rodrigues Kimmo Raatikainen 2 University of Lisbon and LaSIGE - Portugal 2 University of Helsinki - Finland August 29th, 27

2 Motivation The problem To make data items available to all nodes in a MANET posts to a white board SIP/SLP records data collected in field surveys Knowing that Nodes may fail Data should be replicated Storage space is limited It is not possible/desirable to replicate everything at all nodes Nodes have limited batteries Use as few transmissions as possible

3 Related Work SAF, DAFN, DCG On each round, nodes negotiate with their neighbours the items to store. Number and location of the replicas depends of an estimated access pattern No guarantee of geographical distribution Data centric storage Items are stored at the nodes closer to a location extracted from an hash to the item (GHTs) Nodes must be aware of their location Some versions, geographically distribute the replicas

4 Data Distribution Framework Assumptions Nodes are not aware of their location I.e. do not have a GPS device Nodes cannot anticipate the data they will require Goals To place a replica close to every node, to: Reduce access latency Save messages on data retrieval Increase resilience to localised failures (e.g. interference)

5 The Dissemination Algorithm The brief version Data is broadcasted with a counter field (TFS) TFS counts the distance from the transmitting node to the closest copy Before forwarding the message, nodes compute the local minimum of TFS from the redundant transmissions Retransmissions Nodes storing the data set TFS to zero Nodes not storing the data set TFS to minimum + 2

6 The Dissemination Algorithm Some Details Who stores the data? The nodes whose local minimum exceeds a threshold (DbC) stored stored <,data> <,data> < 2,data> <,data>... Who forwards the message? Any node that stores the data 2 Some of the remaining (Pampa) Wait time is dictated by Pampa

7 Pampa An all-purpose broadcast algorithm Selects the nodes required to retransmit a broadcast message Reduces redundancy Saves bandwidth and power Wait time is proportional to RSSI Nodes more distant from the source wait less Nodes do not retransmit if they hear a predefined number of retransmission S st 2nd RSSI & wait time Pampa stretches the hop length

8 Pampa An all-purpose broadcast algorithm Selects the nodes required to retransmit a broadcast message Reduces redundancy Saves bandwidth and power Wait time is proportional to RSSI Nodes more distant from the source wait less Nodes do not retransmit if they hear a predefined number of retransmission S st 2nd RSSI & wait time Pampa stretches the hop length

9 Pampa An all-purpose broadcast algorithm Selects the nodes required to retransmit a broadcast message Reduces redundancy Saves bandwidth and power Wait time is proportional to RSSI Nodes more distant from the source wait less Nodes do not retransmit if they hear a predefined number of retransmission S st 2nd RSSI & wait time Pampa stretches the hop length

10 Pampa An all-purpose broadcast algorithm Selects the nodes required to retransmit a broadcast message Reduces redundancy Saves bandwidth and power Wait time is proportional to RSSI Nodes more distant from the source wait less Nodes do not retransmit if they hear a predefined number of retransmission S st 2nd RSSI & wait time Pampa stretches the hop length

11 Dissemination Example Numbers show TFS of the message. DbC=2

12 Dissemination Example Numbers show TFS of the message. DbC=2

13 Dissemination Example 2 2 Numbers show TFS of the message. DbC=2

14 Dissemination Example 2 2 Numbers show TFS of the message. DbC=2

15 Dissemination Example Numbers show TFS of the message. DbC=2

16 Dissemination Example Numbers show TFS of the message. DbC=2

17 Dissemination Example Numbers show TFS of the message. DbC=2

18 An example of data distribution ns 2 5m 5m nodes 25m Arrows indicate devices that retransmited 7 copies 26 retransmissions

19 Query Two attempts Nodes first broadcast the query with a small TTL The TTL adapts from past experiences If no reply is received, broadcast to all nodes Replies are sent point-to-point Use the route constructed during query propagation (like DSR) Mitigation of a bad distribution If a reply is received from more than DbC hops away, store the item

20 Evaluation Factors Considered Experiments varied: Node density Storage space at each node Free space in the neighbourhood of each node Number of data items advertised Evaluated Average Reply Distance How many hops away are the data items Number of Messages How many messages are required for getting a reply

21 Analytical Properties tfs: hop 2 hops 3 hops 2 hops hop If there is enough space available: All nodes will have a copy at most DbC hops away The majority will have a copy at most DbC+ 2 hops away

22 Expected Distance Assuming: An homogeneous distribution of the nodes Sufficient storage space at the nodes The average distance at which a reply will be found is given by DbC ( i+ ) i= 2 (2i + ) τ (DbC) = (DbC + ) 2 τ(2) =.55, τ(3) =.75 and τ(4) = 2.2

23 Simulation Results Distance of the replies Average reply distance (hops) DbC=2 DbC=3 DbC= Advertised Items Simulations in ns-2, nodes Square defines the theoretical limit for DbC=2

24 Simulation Results Traffic Messages forwarded/query Storage, DbC=2 Storage, DbC=3 Items, DbC=2 Items, DbC=3 Neighbours, DbC=2 Neighbours, DbC= Saturation point ratio.5.5 Changes in node density affect number of forwarders Degrades gracefully with the number of items and storage space

25 Conclusions We presented an algorithm for disseminating replicas of data items that: Geographically distribute the replicas Even when nodes are not aware of their location Creates a number of replicas that depends of the region covered by the network Requires a limited number of retransmissions

26 Future Work Leverage the distribution Improve original distribution Attend to node movement Addition/removal of nodes Apply the algorithms to different applications